1. Field of the Invention
This invention relates to motion dependent video signal processing. More particularly, the invention relates to video standards converters using such signal processing, and to methods of deriving motion vectors representing motion between fields or frames of a video signal.
2. Description of the Prior Art
Video standards converters are well known devices used to convert video signals from one standard to another, for example, from a 625 lines per frame, 50 fields per second standard to a 525 lines per frame, 60 fields per second standard. Video standards conversion cannot be achieved satisfactorily merely by using simple linear interpolation techniques, because of the temporal and vertical alias which is present in a video signal. Thus, simple linear interpolation produces unwanted artifacts in the resulting picture, in particular, the pictures are blurred vertically and judder temporally.
To reduce these problems it has been proposed that video standards converters should use adaptive techniques to switch the parameters of a linear interpolator in dependence on the degree of movement in the picture represented by the incoming video signal.
It has also been proposed for example for the purpose of data reduction in video signal processing, to generate motion vectors from an incoming video signal by a block matching technique, in which the content of a search block in one field or frame is compared with the respective contents of a plurality of search blocks comprised in a search area in the following field or frame, to determine the minimum difference between the contents so compared, and hence the direction and distance of motion if any) of the content of the original search block.
Our European patent specification EP-A2-0 395 268, for example, discloses a motion compensated video standards converter wherein blocks in a first field or frame of a video signal are compared with a plurality of blocks in a following field or frame of the video signal for deriving motion vectors representing the motion of the content of respective blocks. The motion vectors are derived by a motion vector estimator wherein the results of the motion vector estimation are compared with a threshold for checking the validity of the motion vectors being derived. This threshold is chosen from experience and remains unaltered for an entire program sequence.
A problem with this technique is that the threshold is necessarily a compromise chosen to yield the best results with the average scene content of the program sequence. For some video material, therefore, the threshold may be too hih, so too few motion vectors pass the threshold test and the choice of motion vectors available for selection in the subsequent stage is restricted. Conversely, for other material, the threshold may be too low, so too many motion vectors pass the threshold test, and the motion vectors passing to the subsequent stage have an undesirable level of uncertainty.